Mr. Gazzaniga is at his best when he is describing his own research with brain-damaged and split-brain patients – that is, patients in whom the tissue between the left and right brain hemispheres has been severed. He tells of one woman who, “although she was being examined in my office at New York Hospital, claimed we were in her home in Freeport, Maine.” A lesion on her brain had left her so convinced that she was really at home that she subordinated any conflicting information. When Mr. Gazzaniga asked her why, if she really were in her house, there were elevators outside the door, she responded: “Doctor, do you know how much it cost me to have those put in?”

By demonstrating the organic origins of our most basic sense of our selves, such stories can challenge our understanding of personhood, agency and identity.